The present invention relates to a polyester film suitably used for display.
Biaxially stretched polyester films have been used in various applications such as packaging materials, electrical insulating materials, metal-deposited materials, printing plate materials, magnetic recording materials, display materials, transfer materials and window attaching materials because these films are excellent in transparency, dimensional stability, mechanical properties, heat resistance, electric properties, chemical resistance, etc.
In particular, the polyester films have been recently used in various optical applications such as base films for transparent touch panels and prism sheets used in liquid crystal displays as well as base films for protective films provided with functional layers such as antistatic layer, anti-reflective layer and electromagnetic shielding layer which are attached on the surface of a front panel glass of so-called flat displays such as cathode ray tubes, LCD and PDP.
In recent years, there are increasing demands for display panels such as plasma display panels which are used in various electronic devices such as large scale televisions of wall-mounting type, and it is expected that such demands are more and more increased in future.
In the plasma display panels, when a mixed gas of xenon and neon is excited by electric discharge, vacuum ultraviolet rays are irradiated therefrom to excite fluorescent materials from which red, blue and green lights are then respectively emitted. However, it is known that when the exited neon atoms return to the ground state, so-called neon-orange light having mainly a wavelength of about 600 nm is emitted (“Journal of Image Information Media Institute”, Vol. 51, No. 4, pp. 459 to 463 (1997)). Thus, the plasma display panels have such a problem that sufficiently clear red color is not obtainable due to the mixing of red and orange colors. To solve this problem, the plasma display panels are provided on a front surface thereof with a color filter for color correction which contains dyes having a maximum light absorption at 560 to 600 nm, thereby improving absorption to the neon-orange light and allowing the plasma display panels to exhibit a clear red color.
In order to prevent the dyes used in the color filter from being deteriorated upon exposure to ultraviolet light (from display and/or outer atmospheric side), there has been studied such a method of imparting a ultraviolet shielding function to a polyester film itself that is used as a protective film for a front panel of the plasma display, by incorporating an ultraviolet light absorber thereinto.
However, in general, the polyester film containing the ultraviolet light absorber exhibits a strong yellowish color. Therefore, if the polyester film is used as a protective film for a front panel of the plasma display, there arises such a problem that the color correction function of the color filter is adversely affected by the ultraviolet light absorber added thereto.
Further, the color filter using the polyester film made of polyethylene terephthalate suffers from deterioration of dyes contained therein upon exposure to ultraviolet light because the polyethylene terephthalate has a high transmittance in the ultraviolet range. For this reason, there has been studied the method of providing an adhesive layer having an ultraviolet light absorbing effect in the color filter, or the like (Japanese Patent Application Laid-Open (KOKAI) No. 2001-159711, etc.).
However, since the adhesive layer has merely a low ultraviolet light absorbing effect, a considerable amount of the ultraviolet light absorber must be blended therein in order to allow the adhesive layer to exhibit a sufficient ultraviolet light absorbing effect. However, when a large amount of the ultraviolet light absorber is blended in the adhesive layer, there arises such a problem that the adhesion property inherent to the adhesive layer is deteriorated. Therefore, the ultraviolet light absorber is blended in the adhesive layer only in a limited amount.